PC912XDP512F0MPVR datasheet(1233/1348 Pages) FREESCALE

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Chapter 30 Security (S12X9SECV2)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

1235

30.1.5

Unsecuring the Microcontroller

Unsecuring the microcontroller can be done by three different methods:

1. Backdoor key access

2. Reprogramming the security bits

3. Complete memory erase (special modes)

30.1.5.1

Unsecuring the MCU Using the Backdoor Key Access

In normal modes (single chip and expanded), security can be temporarily disabled using the backdoor key

access method. This method requires that:

•

The backdoor key at 0xFF00–0xFF07 (= global addresses 0x7F_FF00–0x7F_FF07) has been

programmed to a valid value.

•

The KEYEN[1:0] bits within the Flash options/security byte select ‘enabled’.

•

In single chip mode, the application program programmed into the microcontroller must be

designed to have the capability to write to the backdoor key locations.

The backdoor key values themselves would not normally be stored within the application data, which

means the application program would have to be designed to receive the backdoor key values from an

external source (e.g. through a serial port). It is not possible to download the backdoor keys using

background debug mode.

The backdoor key access method allows debugging of a secured microcontroller without having to erase

the Flash. This is particularly useful for failure analysis.

NOTE

No word of the backdoor key is allowed to have the value 0x0000 or

0xFFFF.

30.1.5.2

Backdoor Key Access Sequence

These are the necessary steps for a successful backdoor key access sequence:

1. Set the KEYACC bit in the Flash conﬁguration register FCNFG.

2. Write the ﬁrst 16-bit word of the backdoor key to 0xFF00 (0x7F_FF00).

3. Write the second 16-bit word of the backdoor key to 0xFF02 (0x7F_FF02).

4. Write the third 16-bit word of the backdoor key to 0xFF04 (0x7F_FF04).

5. Write the fourth 16-bit word of the backdoor key to 0xFF06 (0x7F_FF06).

6. Clear the KEYACC bit in the Flash Conﬁguration register FCNFG.

NOTE

Flash cannot be read while KEYACC is set. Therefore the code for the

backdoor key access sequence must execute from RAM.

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